Seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the land surface or seafloor. Among other things, seismic data acquisition involves the generation of acoustic waves and the collection of reflected/refracted versions of those acoustic waves to generate the image. This image does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process in the field of seismic surveying.
Mapping subsurface geology during exploration for oil, gas, and other minerals and fluids uses a form of remote sensing to construct two-dimensional or three-dimensional images of the subsurface. The process is known as seismic surveying, wherein an energy source transmits pressure pulses into the earth. These pressure pulses can be reflected by geological interfaces associated with the earth and subsequently recorded at the surface by arrays of detectors.
A contaminant which causes problems in processing and interpreting the subsurface data is a phenomenon known as a ghost reflection. Ghost reflections are caused by the sea-surface operating as a mirror and reflecting up-going pressure-waves. There are source-side ghosts caused by the source energy being reflected back from the sea-surface and there are receiver-side ghosts incident at the detectors as down going surface reflections of the up-going energy from the subsurface.
These ghost reflections do more than complicate the subsurface image with additional waves, the mirror effect which produces the ghost reflections changes the phase of the reflection by 180 degrees such that, in some circumstances, the energy constructively interferes with the desired signal to magnify it and, in other circumstances, destructively interferes with and destroys the desired signal.
Water surface reflections of very long wavelength, low frequency seismic waves, destructively interfere such that there is always a null or notch at zero Hertz. These notches and the associated filtering limit the extent to which subsurface reflections or events can be resolved. This damaging process results in blurred images at best, and at worst, fictitious reflections when the ghost energy lags significantly behind the primary reflection energy. To counter the effect of the notches, conventional seismic surveys are designed to use shallow source and receiver arrays to ensure that the second notch lies at high frequencies. However, the resultant sloped bandpass filter is a pervading problem as it causes seemingly irretrievable damage to low frequency information which is increasingly being sought in the industry to deliver extra value in the interpretation process. Furthermore, a relatively shallow tow of the receivers exposes them to the very noisy-environment that exists close to the sea surface which in itself can be problematic.
Most existing mechanisms to de-ghost seismic data fundamentally rely on recording alternate views of the same data. Data are deliberately acquired with different ghost characteristics so that when combined there is improved signal spectrum coverage and the damaging notches are filled.
There are three commonly used data acquisition variations designed to tackle the ghost problem: slanted streamers; parallel streamers arranged vertically one above another at the same horizontal position, known as “over-under”, and mixtures of different types of detectors (e.g., use of hydrophones with vector-sensors such as accelerometers) at a coincident position, are representative.
The former techniques exploit variations in the recorded ghost effect, which can be processed together to de-ghost the signals. The latter exploits the fact that up-going and down-going energy exhibit different polarity which one type of detector is able to observe, whereas the other type does not. This allows the ghost energy to be removed by careful summation of the two signals based on the fact that the ghost is of opposite polarity in one of the recorded datasets. The slanted streamer array technique deploys (current) standard streamer equipment, whereas the other two data acquisition techniques use an increased number of streamers or sets of duplicate detectors, often called ‘dual sensors’. These therefore increase, doubling at maximum, the amount of data traces recorded.
Once recorded, the data are routinely processed in a computer. The term de-ghosting is used to describe the computer-based step to reduce or remove the ghost effect from the data. Many de-ghosting processes today either involve some adaptive summation to extinguish the ghost by polarity difference, or adaptive summation of differently ghosted waveforms to recombine the primary signal present in both, in essence to infill the spectral notches.
The majority of marine seismic data is acquired with conventionally towed streamers which are equipped with only hydrophones and are all typically towed at the same constant depth for any single survey. Fundamentally, it would be advantageous if methods and systems existed that were able to de-ghost the conventionally acquired marine seismic data. Such a method would not depend on special acquisition geometry or special hardware. Instead, in one example, it would be based on only post-acquisition, specially configured computing hardware and/or software, and therefore be backward compatible with all previously acquired and existing seismic data.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks associated with deghosting seismic data.